Photovoltaic solar panels modules on the market today typically comprise a front cover, a first layer of encapsulant, one or more photovoltaic cells, a second layer of encapsulant, and a layer of insulation adjacent to the second layer of encapsulant on the backside of the solar panel module. The insulation layer is intended to provide electrical insulation for safety, and prevent performance problems such as current leakage or potential short circuits. This insulation layer is generally referred to in the trade as a “backsheet.”
Over the last two decades, this insulation layer, or backsheet, has been primarily constructed as a three layer laminate structure that utilizes: (i) a fluoropolymer exterior layer; (ii) a bi-axially oriented polyester (hereinafter “PET”) core layer; and (iii) either another fluoropolymer layer, or an olefin adhesive layer such clarified polyethylene (hereinafter “PE”) or ethylene vinyl acetate (hereinafter “EVA”) film. This type of backsheet construction is depicted in FIG. 1.
The function of the fluoropolymer layer is solely to provide long term ultraviolet (“UV”) protection of the internal PET layer. Fluoropolymers are well known to provide excellent outdoor weather resistance and long term durability. However, fluoropolymers are fairly expensive components, and under most conditions the useful life of the fluoropolymer layer will far exceed the life of the solar panel.
The PET core layer of the typical backsheet serves two functions: (i) providing excellent insulation characteristics; and (ii) providing excellent dimensional stability. Both properties are critical to successful backsheet performance and need to be maintained over the life of the panel.
The third layer of the current backsheet also provides several functions: (i) it enables a durable bond between the module encapsulant material and the backsheet; (ii) it provides enhanced reflectivity to improve the solar panel module efficiency; and (iii) it also serves as part of the total laminate dielectric material.
Historically the backsheet described above has been made with individual films that are laminated together with various adhesives. The adhesive selection is critical as it has proven to be one of the major weak links in the backsheet and module structure, causing inter layer adhesion issues in the field. Recently the fluoropolymer exterior layer has been applied using a fluoropolymer coating instead of the traditional film. This approach has proven to have two major advantages: first the elimination of one adhesive layer; and, second, the ability to reduce the fluoropolymer layer thickness, thereby reducing the overall cost of manufacturing the solar panel module.
It should be noted, however, that the PET layer, while being an excellent insulator with good dimensional stability, does have some negative characteristics. PET has both poor UV resistance and hydrolysis resistance, which often results in premature failure of the backsheet.
Recently, however, the introduction of backsheets using a PET exterior layer has captured significant market share. These backsheets are made with a special PET exterior layer that has been modified to improve both UV properties and reduce hydrolysis concerns. The interior layer used the same unmodified PET used in the fluoropolymer-based backsheets along with the same olefin adhesive layer. The result is a fairly low cost backsheet that may be adequate when used in some applications. However, this construction is also made with adhesive layers and is subject to interlayer adhesion failures. Even though the PET exterior layer may be modified to perform better than an unmodified PET layer, the reality is that this backsheet is likely to prove to be unsatisfactory over time.
More recently, backsheets based on polyamides have been introduced to the solar panel market. The initial products introduced to the market were based on various layers of polyamides, with the exterior layers being modified with UV absorbers and fillers to provide some facsimile of UV stability. In general, polyamide is not considered for exterior applications due to poor UV stability. These constructions were made with the same lamination process found in other backsheets and are also subject to interlayer adhesion issues. In this regard, long chain polyamides are generally required in backsheet application due to the fact that shorter chain nylons absorb moisture more readily than the long chain polyamides. Short chain nylons can usually absorb up to about 6.5% moisture, which moisture could adversely affect the electrical insulation properties of the backsheet. Although long chain nylons may perform better with the best absorbing only about 2% moisture, long chain nylons are very expensive and add significant cost to the backsheet.
Accordingly, solar panel backsheets currently in use today exhibit several characteristics which leave room for improvement. First, the use of a fluoropolymer layer is costly and is over-engineered in typical solar panel applications. Secondly, on the opposite side of the spectrum, the modified PET or modified polyamide is a high risk for use in the PV system, since it will fail prematurely in many applications causing panels to potentially be unsafe and inefficient. Additionally, a solar panel system that incorporates the use of adhesives is prone to problems in manufacturing as well as subject to premature failure in the field.
Thus, there exists a need for an efficient, durable, weather resistant, and cost effective backsheet used in the construction of solar panel systems. There also exists a need for a solar panel backsheet which eliminates the use of adhesives in the backsheet construction. The need also exists for an efficient and cost effective method for manufacturing such improved solar panel backsheets.